1. Field of the Invention
The present invention relates to a semiconductor device having a function of overheat detection.
2. Description of the Related Art
A semiconductor integrated circuit generates heat due to, for example, an operation of active elements and charges flowing into the semiconductor integrated circuit from outside thereof. A temperature sensing element is thus formed on the same substrate as the semiconductor integrated circuit to control the semiconductor integrated circuit based on a signal from the temperature sensing element, to thereby prevent abnormal operation and breakdown of the circuit due to overheating. As the temperature sensing element, for example, a PN junction is used, and the forward current of the PN junction is used for temperature sensing. To be specific, when a constant current flows through a PN junction in a forward direction, a potential difference appears between the both ends of the PN junction. The potential difference changes depending on, temperature, and hence the potential difference is used as a signal for overheat detection.
In the field of semiconductor integrated circuits, downsizing has been promoted. Because a temperature increase of a semiconductor integrated circuit increases in inverse proportion to an area thereof, a local temperature increase has become intense along with the promotion of downsizing in recent years. As an amount of locally generated heat increases, a temperature sensed by a temperature sensing element increases depending on a substance between a heat generating source and the temperature sensing element, and a positional relationship therebetween. There may be two methods to be employed in order to solve this problem, to thereby correctly estimate the temperature of the heat generating source. Specifically, the heat generating source and the temperature sensing element are formed as close as possible, and a highly heat conductive substance is formed between the heat generating source and the temperature sensing element.
When the temperature sensing element is formed on the same substrate as the heat generating source, problematic operation, namely, parasitic operation occurs as described in Japanese Patent No. 2701824 as a problem. Due to this problem, the heat generating source and the temperature sensing element cannot be formed closely. In view of this, a method disclosed in Japanese Patent No. 2701824 involves forming an insulating film between a heat generating source and a temperature sensing element, to thereby avoid a problem of a parasitic element. Silicon is most commonly used as a substrate and a silicon dioxide film is most commonly used as an insulating film in semiconductor devices. The heat generating source and the temperature sensing element can be formed closely with the silicon dioxide film formed therebetween, which serves as the insulating film. In this case, however, because heat conductivity of a silicon dioxide film is smaller than that of silicon, it takes time for the temperature sensing element to follow the temperature of the heat generating source.
In short, a semiconductor device in which a temperature sensing element is formed on the same substrate as a heat generating source is disadvantageous in distance but is advantageous in heat conduction. On the other hand, a semiconductor device in which a heat generating source and a temperature sensing element are isolated from each other by an insulating film is advantageous in distance but is disadvantageous in heat conduction.